Debonding temporarily bonded semiconductor wafers

ABSTRACT

Described methods and apparatus provide a controlled perturbation to an adhesive bond between a device wafer and a carrier wafer. The controlled perturbation, which can be mechanical, chemical, thermal, or radiative, facilitates the separation of the two wafers without damaging the device wafer. The controlled perturbation initiates a crack either within the adhesive joining the two wafers, at an interface within the adhesive layer (such as between a release layer and the adhesive), or at a wafer/adhesive interface. The crack can then be propagated using any of the foregoing methods, or combinations thereof, used to initiate the crack.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/552,140, “Debonding Temporary Bonded Semiconductor Wafers,” filedOct. 27, 2011, which is incorporated by reference in its entirety. Thisapplication is a continuation-in-part of co-pending U.S. applicationSer. No. 13/085,159 “Debonding equipment and methods for debondingtemporary bonded wafers,” filed Apr. 12, 2011 by Gregory George; whichis a continuation-in-part of U.S. application Ser. No. 12/761,014,“Debonding equipment and methods for debonding temporary bonded wafers,”filed Apr. 15, 2010 by Gregory George et al.; which claims priority toU.S. Provisional Application No. 61/169,753, filed Apr. 16, 2009. All ofthe foregoing are incorporated by reference in their entirety.

BACKGROUND

Embodiments of the present disclosure relate generally to improveddebonding equipment and methods, and more particularly to debondingtemporarily bonded wafers.

Several semiconductor wafer processes include wafer thinning steps. Insome applications the wafers are thinned for the fabrication ofintegrated circuit (IC) devices. Thin wafers have the advantages ofimproved heat removal and better electrical operation of the fabricatedIC devices. Wafer thinning also contributes to a reduction of the devicecapacitance and to an increase of its impedance, both of which result inan overall size reduction of the fabricated device. In otherapplications, wafer thinning is used for 3D-integration bonding and forfabricating through wafer vias.

Wafer thinning is usually performed via back-grinding and/or chemicalmechanical polishing (CMP) of a wafer. CMP involves bringing the wafersurface into contact with a hard and flat rotating horizontal platen inthe presence of liquid slurry. The slurry usually contains abrasivepowders, such as diamond or silicon carbide, along with chemicaletchants such as ammonia, fluoride, or combinations thereof. Polishingthe wafer with the platen using the abrasive slurry thins the wafer,while the etchants polish the surface at the submicron level. The waferis polished until a certain amount of substrate has been removed toachieve a targeted thickness.

For wafer thicknesses greater than 200 μm, the wafer is usually held inplace with a fixture that utilizes a vacuum chuck or some other means ofmechanical attachment. However, for wafer thicknesses of less than 200μm and especially for wafer thicknesses of less than 100 μm, it becomesincreasingly difficult to mechanically hold the wafer and also maintaincontrol of the planarity and integrity of the wafer during thinning. Inthese cases, it is not uncommon for wafers to develop microfractures andbreak during CMP.

An alternative to directly holding a wafer during thinning involvesattaching the device wafer (i.e., the processed wafer) to a carrierwafer for support and then thinning down the exposed opposite surface ofthe device wafer. The bond between the carrier wafer and the devicewafer is temporary and the wafers are separated upon completion of thethinning (or other processing steps).

SUMMARY

Embodiments of the present disclosure include methods and apparatus forproviding a controlled perturbation to an adhesive bond between a devicewafer and a carrier wafer. This controlled perturbation facilitates theseparation of the two wafers without damaging (or reducing the damageto) the device wafer. This controlled perturbation, which can bemechanical, chemical, thermal, radiative, or combinations thereof,within the adhesive joining the two wafers, at an interface within theadhesive layer (such as between a release layer and the adhesive), or ata wafer/adhesive interface. The crack can then be propagated byperforming a controlled debonding of the carrier wafer from the devicewafer while leaving the device wafer intact and undamaged.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross-sectional view of a carrier wafer temporarily bondedto a device wafer, in an embodiment.

FIG. 1B is a cross-sectional view of a carrier wafer temporarily bondedto a device wafer, further showing a wafer support structure and aholdback device for the carrier wafer, in an embodiment.

FIG. 1C is a cross-sectional view showing use of a crack initiator toinitiate a crack for debonding a carrier wafer and device wafer, in anembodiment.

FIG. 1D is a plan view of a crack advancing in a controlled debondingprocess, in an embodiment.

FIG. 1E is a cross-sectional view of a carrier wafer restrained by aholdback device while debonding the device wafer from an adhesive layer,in an embodiment.

FIG. 1F is a cross-sectional view of a carrier wafer debonded from anadhesive layer using a holdback device, in an embodiment.

FIG. 1G is a cross-sectional view of a carrier wafer debonded from adevice wafer using a beveled wafer support structure, in an embodiment

FIGS. 2A-2C are side and plan views of examples of a portion of aholdback device having different shapes, in an embodiment.

FIGS. 3A-3E are plan views (top views) of example tip configurations ofa crack initiator, in an embodiment.

FIGS. 4A-4F are side views of additional examples of tip configurationsof a crack initiator, in an embodiment.

FIGS. 5A-5C are side views illustrating different approaches forengagement of the crack initiator, in an embodiment.

The figures depict various embodiments of the present invention forpurposes of illustration only. One skilled in the art will readilyrecognize from the following discussion that alternative embodiments ofthe structures and methods illustrated herein may be employed withoutdeparting from the principles of the invention described herein.

DETAILED DESCRIPTION

Overview

FIG. 1A is a cross-sectional view of a device wafer 112 temporarilybonded to a carrier wafer 104 by an adhesive layer 108. Embodiments ofthe present disclosure can be used to initiate and propagate a crack inthe adhesive layer 108, and preferably at an interface of the adhesivelayer 108 with either the device wafer 112 or the carrier wafer 104,thereby debonding the two wafers.

Examples of device wafer 112 include silicon wafers, GaAs, GaN wafers,or any other semiconductor wafer, especially those that are thinned areto be thinned to less than 100 μm as part of processing. In otherexamples, embodiments of the present disclosure can be used to de-bondeven thinner and less mechanically robust device wafers 112.Particularly for a device wafer 112 have a thickness of less than 100μm, embodiments of the present disclosure can initiate a crack at awafer interface with the adhesive layer 108 (either the device wafer112/adhesive layer 108 interface or the carrier wafer 104/adhesive layer108 interface), and successfully separate the device wafer from thecarrier wafer 104 without damaging the device wafer, unlike conventionalseparation techniques. As wafers get thinner, it is beneficial toinitiate a crack between the two bonded wafers in a controlled way andexecute a controlled propagation of the crack, thereby separating thewafers in a controlled manner so as to prevent damage to the devicewafer.

In various applications, the device wafer can be as thin as 50 μm oreven as thin as 10 μm. The device wafer 112 typically has a diameterranging from 50 mm to 200 mm to 300 mm or larger.

The carrier wafer 104 is usually made of a non-contaminating materialthat is thermally matched (i.e., having approximately the samecoefficient of thermal expansion) to the device wafer 112. Examples ofcarrier wafer materials include silicon, glass, sapphire, quartz orother convenient substrate materials having mechanical and thermalproperties similar to the device wafer. Preferably, the carrier wafer104 is thicker or is otherwise more mechanically robust than the devicewafer. The carrier wafer 104 then provides mechanical support or addedmechanical robustness to the more fragile (and typically more valuable)device wafer. Providing additional mechanical robustness to the devicewafer is beneficial because, in later stages of semiconductor processingwhen carrier wafers are often used, the device wafer includessemiconductor devices that have been partially or completely fabricated.These device wafers represent a significant financial investment on thepart of the manufacturer. Reducing device wafer damage and/or breakagereduces financial loss.

The adhesive layer 108 is an adhesive that is used to join the carrierwafer 104 to the device wafer 112. The adhesive layer 108 can be formedby using any number of adhesive, polymeric, and/or oligomeric systemsincluding silicones, polyimides, acrylates and a variety ofthermoplastics. Some systems used for the adhesive layer 108 includemultiple layers, which can perform functions beyond just bonding the twowafers. In some examples, in addition to an adhesive, the adhesive layer108 includes a release layer that is adjacent to one of the wafers 104or 112 that facilitates separation of the adhesive from a wafer. Inother examples, the adhesive layer 108 includes a primer layer adjacentto one of the wafers 104 or 112 that improves the adhesion between theadhesive and the wafer. In still other examples, the adhesive layer 108include both of these types of layers, or multiple layers of adhesiveshaving different mechanical (e.g., modulus, fracture toughness, glasstransition temperature), thermal, or chemical properties. Using multiplelayers of different adhesives, primers, and/or release layers can tailorthe properties of the adhesive layer 108, thereby facilitating thecontrolled separation of the wafers 104 and 112 from each otherregardless of the particular geometries and mechanical properties of thewafers.

The thickness of the adhesive layer 108 is generally a function of theparticular adhesive system applied to the wafer, the amount applied (asinfluenced by the mechanical properties of the adhesive system and thewafers 104 and 112), the topography of features on the device wafer, andother similar factors. In thinning applications, the processed surfaceof the device wafer 112 (i.e., the surface with topography) typicallyfaces the adhesive layer 108, and the unprocessed or backside of thedevice wafer is exposed for thinning. In one embodiment, the thicknessof the adhesive layer 108 is sufficient for providing an approximately10 μm to 15 μm band of adhesive over surface features of the devicewafer 112. Example surface features on a device wafer include, but arenot limited to, C4 bumps, micro bumps and other electrically activefeatures of a semiconductor device.

Other factors that can influence the thickness of the adhesive layer 108include the types of layers used in the adhesive layer, the fracturetoughness of the layers, the viscosity and surface tension of theadhesive components applied to the wafer, the adhesive applicationmethod used, and other similar factors. The adhesive layer preferablyprovides adhesion that is particularly strong with respect to shearstress applied between the adhesive layer and the wafers 104 and 112(i.e., in a direction parallel to the adhesive layer) and is not asstrong with respect to a normal stress applied to separate the twowafers.

The adhesive layer 108 is applied so that the exposed surface of thedevice wafer 112 and the exposed surface of the carrier wafer 104 (i.e.,the two surfaces that do not contact the adhesive layer) are parallelwithin, in some examples, approximately 4 μm In other examples, thedevice wafer 112 and the carrier wafer 104 are parallel to withinapproximately 1 μm, or to less than 1 μm of total thickness variationacross the two wafer stack.

Debonding System

FIG. 1B illustrates one high level embodiment of a system 100 used fordebonding the carrier wafer 104 from the device wafer 112 by initiatinga crack (or craze) at an edge of the adhesive layer 108 proximate to awafer/adhesive layer interface using a controlled mechanical, chemical,thermal, or radiative perturbation. In addition to the elementsintroduced in FIG. 1A, the system 100 includes a wafer support structure116, a holdback device 120, and a crack initiator 124.

The wafer support structure 116 provides structural support for thecarrier wafer 104 and/or the wafer stack 104/112. The wafer supportstructure 116 may also provide a structure to which the holdback device120 is attached. A second wafer support structure may be used to supportthe device wafer 112. For clarity, the second wafer support structure isnot shown in FIG. 1B. One design is based on the mechanical debonderdescribed in FIGS. 30-41 of U.S. application Ser. No. 13/085,159, whichis incorporated herein by reference. The wafer support structure 116 inFIG. 1B is the flex plate (item 253) of the mechanical debonder, and thesecond wafer support structure (not shown in FIG. 1B) is the porousvacuum chuck (item 256) of the mechanical debonder.

The crack initiator 124 is used to initiate, and/or propagate, a crackbetween the adhesive layer 108 and either the carrier wafer 104 or thedevice wafer 112 starting at an edge of the adhesive layer. This isillustrated in FIG. 1C. The crack is used as part of a controlleddebonding process of the wafers 104 and 112.

In a controlled debonding process, the crack advances in a controlledfashion, for example approximately as a line, as shown in FIG. 1D. FIG.1D shows a plan view (top view) of the device wafer 112. The crackinitiator 124 initiates the crack at point 150. The crack thenpropagates right to left in FIG. 1D. The crack propagation may be theresult of normal stress applied to the two wafers to separate themand/or further use of the crack initiator 124. The dashed lines 151-155show crack propagation over time. Each dashed line represents theleading edge of the crack at a different point in time. For each dashedline, at that point in time, the two wafers are separated to the rightof the dashed line and they are still bonded together to the left of thedashed line. In this example, the leading edge of the crack remainssubstantially straight as the crack propagates. Such a controlleddebonding can be achieved by the mechanical debonder described in FIGS.30-41 of U.S. application Ser. No. 13/085,159, for example.

The crack does not always have to be initiated at the same point. Forexample, the crack initiator 124 may be positionable to different pointsalong the edge of the adhesive layer. It may be that the crack initiator124 can be repositioned relative to the wafer stack, or that there aremultiple crack initiators at different locations, or that the waferstack can be repositioned relative to the crack initiator. In oneapproach, the wafer stack can be rotated relative to the crackinitiator, thus allowing the crack initiator to try different locationsfor crack initiation.

There may also be multiple initiations before actual crack initiation.For example, the crack initiator may engage the same point multipletimes before crack initiation, applying more pressure each time orapproaching from a different angle or direction. Alternately, the crackinitiator may engage multiple different points before crack initiation.In one process, the crack initiator engages and disengages at a point,the wafer stack is rotated slightly, the crack initiator then engagesand disengages the new point, and so on until crack initiation.

Returning to FIG. 1, in some embodiments, the wafer support structure116 is planar and rigid. For example, if the wafer support structure 116is a portable chuck that is used to hold the bonded wafers during CMP,then the wafer support structure 116 preferably is planar and rigid. Ifthe same wafer support structure 116 is used during debonding, then thedevice wafer 112 is “peeled” away from the carrier wafer 104, as shownin FIG. 1E. The crack initiator 124 starts the peeling. The holdbackdevice 120 mechanically restrains the carrier wafer 104 during thedebonding process.

While the wafer support structure 116 can be planar and/or rigid, someexamples of the wafer support structure are flexible, as shown in FIG.1F. One example is the mechanical debonder described in FIGS. 30-41 ofU.S. application Ser. No. 13/085,159. In that embodiment, the wafersupport structure is a flex plate 176. The device wafer 112 is securedby its own chuck 170 (e.g., a porous vacuum chuck, since the devicewafer is so thin). The device wafer 112 is debonded onto tape 168 heldby tape frame 164.

The flex plate 176 can be used to expose the adhesive layer 108 (or morespecifically, an adhesive layer/wafer interface) to the crack initiator124 by, for example, flexing the carrier wafer 104 away from the devicewafer 112. In addition to exposing the adhesive layer 108 by flexing thecarrier wafer 104, the flexing of the carrier wafer 104 also exerts anapproximately normal stress on the adhesive layer. This normal stresscaused by flexing the carrier wafer 104 via the flex plate 176 canfacilitate or initiate crack growth either alone or in combination withthe crack initiator 124. Furthermore, using the holdback device 120 andthe flex plate 176 to exert a stress on the carrier wafer 104 and theadhesive layer 108, the holdback device can be used to control thepropagation of the crack tip so that the carrier wafer is separated fromthe device wafer 112 in a controlled release.

The holdback device 120 is used to physically secure the carrier wafer104 to the flex plate 176, thereby providing additional mechanicalstability and security to the carrier wafer during separation from thedevice wafer 112. In this case, the carrier wafer 104 can also beattached to the flex plate via a vacuum, one or more clamps, or otherreleasable connection. By providing this physical security to thecarrier wafer 104, the holdback device 120 helps prevent damage to oneor both of the carrier wafer and the device wafer 112 during waferprocessing or during separation of the wafers.

The holdback device 120 may also be used to apply a stress to thecarrier wafer 104 during separation of the carrier wafer from the devicewafer 112. In FIG. 1F for example, the holdback device 120 can apply astress that is approximately perpendicular to the plane of the carrierwafer 104 that, as described above, flexes the carrier wafer. Theresulting strain of the carrier wafer 104 produced by this normal stresscan expose the adhesive layer 108 for better access by the crackinitiator 124, or can initiate the crack between the adhesive layer andone or both of the wafers 104 and/or 112.

Once the crack is initiated, whether through action of the holdbackdevice 120 or independent from it, a stress applied by the holdbackdevice on the carrier wafer 104 is used to provide a controlledseparation of the carrier wafer from the device wafer 112. For example,once the crack is initiated, the propagation of the leading edge of thecrack can be controlled by providing controlled continued stress. Oncethe crack propagates to the target location or interface, the stressand/or the strain rate can be reduced to maintain the trajectory of thecrack along the target path.

FIG. 1G shows the use of a beveled wafer support structure 160 for thedevice wafer 112. In this example, the device wafer 112 is debonded ontotape 168 held by tape frame 164. The beveled shape allows easier accessto the edge of the adhesive layer for crack initiation. For example, thewafer stack can be engaged beginning on the right side of the tape 168.It can then be “rocked” from right to left across the beveled surface.This will apply a similar stress as the peeling motion of FIG. 1F.

The holdback device 120 includes a wafer contact surface 122 thatcontacts the carrier wafer 104, thereby securing it or applying a stressor strain to it, as described above. Turning now to FIGS. 2A-2C, theshape of the wafer contact surface 122 can be adapted to the particularstructural or mechanical properties of the carrier wafer 104. FIGS.2B-2C show a plan view in addition to a side cross-sectional view. Asshown in FIGS. 2A-2C, the wafer contact surface 122 can include a slopedbackstop 204, a stepped backstop 208, or a scalloped backstop 212. Thesedifferent backstop profiles can be selected according to the amount ortype of contact desired with the carrier wafer 104. In the plan view,the holdback device can have a straight backstop (as shown in FIG. 2B)or a curved backstop that conforms to the wafer shape (as shown in FIG.2C). The conformal backstop has a larger contact area with the carrierwafer 104 at the wafer edge compared to the straight backstop, which mayonly contact a portion of the perimeter edge of the wafer. These threeexample backstops are presented only as examples. Other profiles orbackstop shapes of the wafer contact surface 122 are possible and thedifferent plan and side views may be mixed. For example, the steppedbackstop 208 of FIG. 2B may be combined with the conformal plan view ofFIG. 2C.

Crack Initiator Apparatus

As described above, the crack initiator 124 is used to initiate and/orpropagate a crack starting at an edge of the adhesive layer, andpreferably located between the adhesive layer 108 and either the carrierwafer 104 or the device wafer 112. The crack is used as part of acontrolled debonding process of the wafers 104 and 112. In a controlleddebonding process, the leading edge of the crack advances approximatelyin the shape of a line, as shown in FIG. 1D. The controlledperturbations from the crack initiator 124 can be provided by using, forexample, mechanical, thermal, chemical, and/or radiative means. Inanother aspect, the crack initiator 124 can be used to conductelectrical charge from the device wafer 112, thereby reducing the riskof an electro-static discharge damaging the integrated circuits on thedevice wafer.

In one example, the crack initiator 124 initiates the crack by simplyimpinging at an interface between the adhesive layer and either thedevice wafer 112 or the carrier wafer 104. This mechanically introducesa crack at this interface. This mechanical perturbation can becontrolled by the shape of the crack initiator 124 and by controllingthe force, velocity, angle of perturbation delivery, location of theperturbation, added types of perturbation (thermal, chemical, etc.),depth of penetration, and/or acceleration of the crack initiator.Different tip configurations are discussed in further detail below.

In examples in which the adhesive layer 108 includes viscoelastic layersor components, the rate at which the crack initiator 124 penetrates theadhesive material layer can influence the initiation, propagation, andcharacteristics of the crack. For example, if the material(s) used forthe adhesive layer 108 are strongly viscoelastic, the crack initiatormay introduce a controlled mechanical perturbation at a relatively highvelocity and/or acceleration to introduce a crack into the adhesivelayer or at the edge of the adhesive layer/wafer interface in the glassyregime of the material. Providing a low velocity and/or low accelerationmechanical perturbation to a highly viscoelastic adhesive layer 108 inthis example could either fail to crack the adhesive layer at theinterface or create a crack with a blunted crack tip (thus reducing thestress concentration at the crack tip). This in turn can increase theenergy needed to propagate the crack, thereby increasing the stresses,and the risk of damage, on the carrier and device wafers 104 and 112.

In another example, the crack initiator 124 can provide a mechanicalperturbation at the edge of the adhesive layer 108/wafer 104 or 112interface using an ultrasonic or megasonic frequency, such as thatproduced by a piezo-electric transducer. The piezo-electric transducercan be integrated into the crack initiator 124 as a separate device, ora tip of the crack initiator can itself be a piezo-electric transducerthat is actuated by a remotely located controller. The ultrasonicmechanical perturbation can facilitate crack initiation by, for example,perturbing the adhesive material in a regime where the material is lessductile or less able to absorb energy provided by the crack initiator124.

In yet another example, the crack initiator 124 initiates a crack byproviding a controlled thermal perturbation to the adhesive layer. Insome examples, the crack initiator 124 either conducts heat from aremote heat source or includes a heating element. In either case, thecrack initiator 124 can deliver the heat to the adhesive layer, therebyproviding a controlled thermal perturbation to the adhesive layer 108that can be used to initiate a controlled release of the carrier wafer104 from the device wafer 112. The controlled thermal perturbation caninitiate this controlled release of the wafers by, for example, meltinga portion of the adhesive layer 108, raising a portion of the adhesivelayer above a glass transition temperature of a material used as one ormore layers of the adhesive layer, or burning a portion of the adhesivelayer.

In still yet another example, rather than introducing a controlledthermal perturbation by heating a portion of the adhesive layer 108, thecrack initiator 124 can introduce a controlled thermal perturbation bycooling the adhesive layer using an integrated or remotely locatedcooling source. In one embodiment of this example, the crack initiator124 can cool an adhesive layer 108 that includes a viscoelastic materialbelow a glass transition temperature, thereby making the material lesscompliant and/or reducing its fracture toughness. This chilled portionof the adhesive layer can be cracked more easily compared to thematerial above its glass transition temperature. Methods of coolinginclude using the crack initiator 124 to deliver a refrigerant orcryogen (such as an ether, liquid nitrogen or solid carbon dioxide) tothe adhesive layer 108 and/or using a remotely cooled crack tipinitiator as a head sink to conduct heat from the adhesive layer.

In other embodiments, even a non-viscoelastic material can have areduced fracture toughness or increased brittleness when sufficientlycooled. In some examples, cooling non-viscoelastic materials used in theadhesive layer 108 reduces elastic modulus, which can increase stressconcentration at the crack tip, thereby lowering fracture energy. Instill other embodiments, cooling the adhesive layer 108 can be combinedwith a controlled mechanical perturbation to initiate a crack byincreasing the brittleness (or reducing the fracture toughness) of theadhesive layer and then mechanically perturbing it.

In yet another example, the crack initiator 124 initiates a crack byproviding a controlled chemical perturbation to the adhesive layer. Inone embodiment, the chemical used to provide the perturbation isselected responsive to the components of the adhesive system. That is,the chemical in this embodiment is selected to be a solvent or swellingagent for the adhesive layer 108, or one or more layers within theadhesive layer, such as release layers or primer layers. By providing achemical that weakens the adhesive properties of the adhesive layer 108or the structural integrity of the layer (or layers therein), thecontrolled release of the carrier wafer 104 from the device wafer 112can be performed by applying a stress or strain to the carrier waferusing the holdback device.

In one embodiment, the controlled chemical perturbation can be deliveredusing the crack initiator 124 as a conduit for a fluid chemical that isthen presented to the adhesive layer 108. The chemical can be drawn intoa pre-existing crack by capillary action or by being injected by thecrack initiator 124, thereby facilitating crack growth. In anotherembodiment, the chemical can be provided to or all of the exposedadhesive layer 108 that is roughly concentric with the device wafer 112.This can then weaken the interface around the circumference of theadhesive layer 108, enabling a controlled debonding using the holdbackdevice 120 to exert a stress on the carrier wafer 104, thereby pullingthe carrier wafer away from the weakened adhesive.

In still yet another example, the crack initiator 124 can include aradiation delivery device that can be used to irradiate the adhesivelayer 108 as part of initiating a crack. The radiation delivered can beselected in combination with the adhesive material used, and can includeinfrared, visible or ultraviolet radiation (delivered from a sourceusing e.g. an optical fiber), as well as microwave radiation or otherfrequencies of electromagnetic radiation. In one embodiment, infraredradiation can be used to provide heat energy to the adhesive layer 108to facilitate debonding. In another embodiment, ultraviolet radiation isprovided to the adhesive layer 108 to facilitate chain scission of anoptically active adhesive (e.g., polymeric or oligomeric systemsincluding methacrylates), thereby reducing adhesion and reducing theamount of energy needed to separate the carrier wafer 104 from thedevice wafer 112. As described elsewhere, the delivery of radiation canbe combined with other mechanisms for initiating and propagating a crackat the adhesive layer 108.

Crack Initiator Tips

FIGS. 3A-3E are plan views (top views) of a variety of cross-sections ofthe crack initiator 124, each initiator shown having a different tip.The crack initiator 124 and its tip can be selected based on thefracture properties of the adhesive layer 108 at the wafer interfaceand/or other crack initiation technique(s) used to initiate andpropagate a crack, such as those described above. As shown, the crackinitiator includes a shaft 302 that can be connected to a rounded tip304, a pointed tip 308, a planar tip 312, a serrated tip 316, or aconcave arcuate tip 320. The shafts 302 are all shown as the same widthin FIGS. 3A-3E, but the widths may also vary. For example, a narrowercrack initiator may be used if the purpose is just to initiate thecrack. A wider crack initiator may be used if it will also be used tofurther propagate the crack after initiation.

In one embodiment, the pointed tip 308 can be selected to initiate acrack in, for example, a brittle adhesive layer 108. In anotherembodiment, the pointed tip 308 can be used to initiate a crack in theadhesive layer in combination with a cooling thermal perturbation thatincreases the brittleness (or reduces the fracture toughness) of theadhesive layer 108 before or during crack initiation. In anotherembodiment, the rounded tip 304 can be selected to accommodate, forexample, a chemical or radiation delivery system (such as a tube,channel or capillary) that provides a chemical or radiationalperturbation to the adhesive layer 108. In still yet another embodiment,the serrated tip 316 can be selected to initiate multiple cracks in theadhesive layer 108. These examples are provided only as illustrationsfor some of the reasons to select a particular tip shape of the crackinitiator 124.

In some examples, the crack initiator 124 is configured to initiate acrack at a target location within the adhesive layer 108. The benefit ofthis is that, as the diversity of adhesive systems used in the industryincreases and/or the number of sub-layers in the adhesive layer 108increases, introducing a crack in the adhesive layer at a targetlocation can improve the ability to separate the carrier wafer 104 fromthe device wafer 112 without damaging one or both of the wafers.

For example, it can be beneficial to initiate a crack proximate to thedevice wafer 112 for cases in which the adhesive layer 108 includes arelease layer adjacent to the device wafer. The release layer reducesthe energy needed to separate the carrier wafer 104 from the devicewafer 112. Reducing the energy of separation can increase the likelihoodthat the device wafer 112 is separated from the carrier wafer 104without damage.

Initiating a crack at or near a target location within the adhesivelayer 108 can be facilitated by, in some examples, using a crackinitiator 124 that is configured to introduce a mechanical, thermal,chemical, or radiative perturbation at or near the target location. Abenefit of this is that, for adhesive systems that include a releaselayer, a perturbation would be more effective for a controlled debondingof the wafers when delivered proximate to the release layer, therebyinitiating a crack at or near an interface that needs less energy todebond.

FIGS. 4A-4F are side views of a variety of cross-sections of the crackinitiator 124, each initiator shown having a different tip. As shown,the crack initiator includes a shaft 302 that can be connected to a flattip 404, a wedged tip 408, a pointed tip 412, a concave arcuate tip 416,a rounded tip 420 or a scalloped tip 424. Any of these side view crosssections can be combined with the top view cross sections of FIGS.3A-3E.

The crack initiator 124 can be used in different ways. In FIG. 5A, afirst face of the crack initiator 124 is placed proximate to surface ofthe carrier wafer 104, thereby placing the wedge-shaped tip proximate tothe wafer/adhesive interface. This configuration then delivers theperturbation (whether mechanical, thermal, chemical, radiative, orcombinations thereof) at or near the target interface that is intendedto be used to separate the carrier wafer 104 from the device wafer 112.

The angle at which the crack initiator 124 is presented to the adhesivelayer 108 can also facilitate providing a perturbation proximate to atarget location of the adhesive layer. In FIG. 5B, the crack initiator124 is presented to the adhesive layer 108 at an acute angle withrespect to a wafer from which the adhesive layer is to be debonded from.One benefit of this is that the crack initiator 124 can be presented tothe adhesive layer free from entanglements with other components of thesystem, such as a dicing frame, dicing tape, the holdback device, and/orother components of the system.

The crack initiator 124 may also be presented to the adhesive layer 108using a flexible or compliant shaft or other support member. Using aflexible support member has the advantage of allowing the crackinitiator 124 to be guided to a target location by other features of thesystem. In FIG. 5C, the crack initiator 124 can be guided to awafer/adhesive layer interface by flexing the support member 126 whiletranslating the crack initiator 124 towards the interface. Flexing thesupport member 126 maintains contact with the carrier wafer untilcontacting the adhesive layer adjacent to the wafer surface.

The foregoing description of the embodiments of the invention has beenpresented for the purpose of illustration; it is not intended to beexhaustive or to limit the invention to the precise forms disclosed.Persons skilled in the relevant art can appreciate that manymodifications and variations are possible in light of the abovedisclosure.

What is claimed is:
 1. A system for debonding a device wafer from acarrier wafer, the device wafer temporarily bonded to the carrier wafer,the system comprising: a wafer support structure configured for holdinga wafer stack comprising the carrier wafer temporarily bonded to thedevice wafer by an adhesive layer; a crack initiator configured toinitiate a crack in the adhesive layer by introducing a controlledperturbation with a tip of the crack initiator near an edge of theadhesive layer; and a holdback device comprising a wafer contactsurface, the wafer contact surface comprising: a first portion parallelto a plane of the carrier wafer, the first portion confronting a firstside of the carrier wafer, the first side of the carrier wafer bonded tothe device wafer; and a backstop connected to the first portion andconfigured to contact a circumferential edge of the carrier waferadjacent to the first side of the carrier wafer, wherein the firstportion of the holdback device applies a controlled force to the firstside of the carrier wafer, the controlled force flexing the carrierwafer away from the device wafer, the flexing used to controlpropagation of the crack during debonding.
 2. The system of claim 1,wherein the device wafer has a thickness of less than 50 μm.
 3. Thesystem of claim 2, wherein the device wafer has a thickness of less than10 μm.
 4. The system of claim 1, wherein the wafer stack has a totalthickness variation of less than 1 μm.
 5. The system of claim 1, whereinthe wafer support structure is a vacuum chuck that holds the wafer stackboth during processing of the device wafer and during debonding of thedevice wafer from the carrier wafer.
 6. The system of claim 5, whereinthe vacuum chuck holds the wafer stack during thinning of the devicewafer.
 7. The system of claim 1 wherein the wafer support structure isflexible to allow flexing of the carrier wafer away from the devicewafer during debonding.
 8. The system of claim 1 wherein the crackpropagates with a substantially straight leading edge.
 9. The system ofclaim 1 wherein the crack initiator is further configured to propagatethe crack after initiation.
 10. The system of claim 1, wherein the crackis initiated at an interface between the adhesive layer and the carrierwafer.
 11. The system of claim 1, wherein the crack is initiated at aninterface between the adhesive layer and the device wafer.
 12. Thesystem of claim 1, wherein the crack initiator provides at least acontrolled mechanical perturbation to the adhesive layer to initiate thecrack.
 13. The system of claim 1, wherein the crack initiator furthercomprises a chemical delivery device used to provide a controlledchemical perturbation to the adhesive layer to initiate debonding of thedevice wafer from the carrier wafer.
 14. The system of claim 1, whereinthe crack initiator further comprises a thermal energy delivery deviceused to provide a controlled thermal perturbation to the adhesive layerto initiate debonding of the device wafer from the carrier wafer. 15.The system of claim 1, wherein the crack initiator further comprises aradiation delivery device used to provide a controlled radiationperturbation to the adhesive layer to initiate debonding of the devicewafer from the carrier wafer.
 16. The system of claim 1, wherein thecrack initiator further comprises means for providing a controlledperturbation to the adhesive layer to initiate debonding of the devicewafer from the carrier wafer.
 17. The system of claim 1, wherein thebackstop is a sloped surface.
 18. The system of claim 1, wherein thebackstop is a scalloped surface.
 19. The system of claim 1, wherein thebackstop is connected to the first portion at an angle greater than 90°.20. The system of claim 1, wherein the backstop is a conformal surfaceconforming to the circumferential edge of the carrier wafer.